This invention relates to a product for use in reinforcing structures and a method for attaching the product to the structure and in particular to reinforce concrete walls using carbon fiber material with epoxy adhered to the carbon fiber material. The invention further includes a rigidified mesh of carbon fiber material designed for adherence to a structural element.
Walls constructed of concrete blocks are well known in the field of construction and have been extensively used for both above ground and basement walls. Such concrete walls constructed in this manner are generally capable of supporting residential and light commercial structures and are relatively inexpensive to manufacture and repair.
In order to construct a concrete wall, individual blocks are laid end to end and successive rows or courses are stacked thereon. Mortar between each adjacent block and row secures the wall together. These walls are such that they have excellent compressive strength to support structures placed upon them. However, these walls are inherently weak with respect to lateral loads and are particularly susceptible to cracking from water pressure. This inherent weakness of concrete walls is attributable to the structural characteristics of the concrete walls themselves and the mortar joints at which they are connected. Walls constructed in this manner are relatively strong in compression and are thus well suited for supporting overlying structures. However, both the concrete material and particularly the mortar joints are weak in tension, and when subjected to a tensile force, they tend to separate relatively easily.
Water penetrating deeply into the soil adjacent a basement wall causes substantial lateral movement of the expanding soil against the wall. Over a period of time, block or concrete walls develop diagonal cracks at the ends and vertical cracks near their centers. Such cracks can admit water under pressure from the surrounding soil and, if left untreated, can progressively widen and eventually facilitate collapse of the entire structure with resultant damage to the structure supported on it. In addition to developing such cracks, concrete walls typically either bow inwardly and such bowing or tilting steadily worsens with the weight of the overlying structure. The water pressure exerts a compressive force at the outer end, therefore, basement wall cracks tend to develop on the inside of such walls.
One of the traditional methods of repairing the leaks and cracks and relieving the external pressure is to drill holes and provide for channeling of the water away on the inside. Yet another method for repairing cracks and leaks is to inject an epoxy resin into the cracks. Although these methods will prevent further water from entering the cracks they do not bind the concrete walls and prevent further cracking or bowing of the concrete walls.
Yet another means of correcting the cracks in the walls is to use fiberglass cloth with epoxy or polyester resin. Fiberglass has good tensile properties and can carry the load on the interior of the basement walls that is in tension. However, one of the major drawbacks with this method is that mixing the epoxy or polyester and wetting out the fabric is time consuming and messy.
The use of carbon fiber bonded plates for structural application has been an area of study for many years. The cost of the carbon fiber material has not allowed practical applications to keep up with academic evaluations. Carbon fiber plates have been studied as an external reinforcement by the Swiss Federal Testing Laboratories and is documented in a paper written in 1995. [Meier, U,; Winsitorfer, A. Retro-fitting of Structures through external Bonding of CFRP Sheets. Non-Metallic Reinforcement of Concrete Structures, Ghent, (1995), pages 465-472]. In this case it was xe2x80x9cused to repair a defective structure, to allow for increases in applied load, or to allow modification for changes in use.xe2x80x9d
Another experiment was carried out in the use of carbon fiber strips to reinforce masonry structures to resist earthquakes. [STEINER, W. Strengthening of structures with CFRP strips. Advanced Composite Materials and Structures, Montreal, 1996, pp407-417.
The use of carbon fiber plates for reinforcement and repair of concrete structures is well documented and known by those skilled in the art. In recent years, this technology, which has been well documented in the literature, has, with lowered cost of carbon material, become economically feasible to apply this technology to concrete walls which are reinforced using precut strips of carbon fiber. This prevents the walls from cracking or collapsing. However, precut carbon fiber strips have to be cleaned and roughened, commonly done through sanding, to provide mechanical adhesion with the walls. The sanding process is not only time consuming, but is completely dependent on the skill of the operator sanding the surface of the strip. Sanding also may not remove oil or waxy materials and may spread such contaminants with a detrimental affect on bonding. This results in extra cost in transporting and storing the precut strips.
The strips of carbon for these prior systems must also be narrow to insure that air pockets are not trapped under the carbon fiber strips. Inspection for air pockets is costly and difficult.
The strips being of high fiber density provides a very stiff strip which greatly differs in stiffness from the masonry that it is reinforcing this causes an xe2x80x9cedge effectxe2x80x9d stress between the reinforced and un-reinforced masonry. The stiffness variation at the edge of the strip has been shown to cause cracking at the junction for the reinforced to un-reinforced masonry due to shear at this large change in relative stiffness.
Also, the thickness of normal pultrusion or carbon plates provides a structure which is much too stiff for the bonding characteristics of the adhesive or the surface strength of the masonry structure. That is, the reinforcement is of no use if it pulls loose from the surface to which it is bonded. This loosening can be due to the reinforcement being too strong and pulling loose due to unequal thermal expansion, this condition is aggravated by the reinforcement being too stiff.
The carbon strips, which are very stiff, can come loose from the structure that it is reinforcing. Generally the bonding material and the surface of the structure being reinforced is much weaker and has a significantly lower modulus of elasticity than that of the carbon fiber. This sudden loosening starts with a high stress causing a crack at an air void or other high stress area; this initial crack can propagate due to a high stress at the crack and lack of elongation in the carbon fiber material.
The use of a plate also blocks moisture flow through the masonry structure thus capturing moisture behind the plate. This moisture rich area tends to weaken the masonry and masonry to adhesive bond area. In areas of frost this moisture allows frost action to work on the bond with deleterious freezing and expansion of the moisture; this action can cause delamination.
With the limitations of the prior art in mind, it is an object of the present invention to provide an article for reinforcing a structure element to effectively resist bending or other lateral forces applied to the structure element.
Another object of the present invention is to provide an article that does not require any sanding to provide mechanical adhesion in order to attach the article to the structural element and one which will remain sufficiently clean at a job site.
It is yet another object of the present invention to provide an article that prevents air pockets from forming, that inhibits micro-crack propagation, and prevents thick glue areas from developing.
It is yet another object of the present invention to provide a method whereby an article for use in reinforcing is firmly adhered to the structural element, thereby reinforcing the structural member.
It is yet another object of the present invention to use existing materials to apply uniform pressure to firmly adhere the article to the structural element, thereby resulting in a strong reinforced structural member.
Another object of the present invention is to provide a product and method which is economical, efficient in operation, and capable of a long operating life.
It is another object of the present invention to provide a method for adhering a breathable strip and adhesive to allow moisture migration through the bonded reinforcement.
It is another object of the present invention to provide a method for applying a reinforcement structure that is wider than a strip to minimize the xe2x80x9cedge effectxe2x80x9d stresses at the boundary between the reinforced and un-reinforced structure.
It is also an object to provide reinforcement through spacing and choice of fiber type to provide a better match between the reinforcement and the structure. This provides a strip which does not overpower the bonding material.
Another object is to taper the edges of the sheet to reduce stress concentrations along the edges of the strip.
Another object is to provide a reinforcement that, if it fails, will fail in a progressive manner thus providing a visual warning that failure is occurring.
Another object is to provide a method of applying reinforcements to the wall which hold the reinforcement in place until the bonding is completed for a neat, strong and workmanlike reinforcement.
Another object is to provide a method of manufacturing a grid where the lateral fibers are woven or wrapped to the longitudinal fibers thus tying the fibers together to allow the stiff longitudinal fibers to carry the load and the elastic lateral fibers to deform to spread the loading more evenly across the glue and supported structure.
In accordance with the preferred embodiment of the present invention, an article (a reinforcing member) and method for reinforcing structural elements, such as concrete walls, support beams and the like, are provided.
The article in accordance with the present invention comprises a carbon fiber strip with an exposed roughened surface. In order to make the article, epoxy resin is applied to carbon fibers. The epoxy resin is allowed to permeate the thickness of the carbon fibers. Due to the uneven surface of the carbon fibers, a thin layer of epoxy forms on a top or exterior of the carbon fiber. A plastic cover sheet is then placed on top of the carbon fiber. Carbon fiber material with the epoxy and the plastic fiber is then subjected to high heat and pressure to cure the epoxy thereby forming the carbon fiber into a rigid sheet, with an adhered cover sheet, that can be cut into strips. At the job site, the cover sheet is readily removed and the resultant sheet of carbon fiber will have a roughened surface defined by the epoxy resin where it had adhered to the cover sheet. The cover sheet keeps out greases and oils that sanding may not remove.
In an alternate embodiment of the present invention, the article comprises a rigidified carbon fiber mesh tape. The mesh tape is comprised of a number of carbon fibers woven together to form a rigidified matrix. When bonded to a structural element, the bonding agent flows through the mesh, eliminating air pockets and thick glue areas. Moreover, propagation of micro-cracks formed between fibers is limited to the spaces between fibers and cannot propagate along the mesh tape.
The mesh tape when used with a breathable adhesive will not trap moisture behind the reinforcement.
The mesh tape since air entrapment is not an issue allows a wide sheet to be used. The wider sheet minimizes the structural changes between the reinforced and un-reinforced transition in the structure.
The edges of the mesh tape can have wider fiber spacing to feather the stiffness of the tape so not to cause a large stress concentration at the junction of the reinforced structure to the un-reinforced structure.
The method of adhering the article to a structural element comprises the steps of applying a second epoxy resin to the structural element; adhering the article to the second epoxy resin; allowing the second resin to cure while applying pressure to the article and structural element. Pressure may be applied through use of an overlying plastic sheet, the edges of which are sealed with the help of an adhesive to the structural element. A uniform pressure is applied with the help of an external vacuum pump and the vacuum is applied until the epoxy is cured and the article is firmly fixed to the structural element.